When a valve-acting metal is sintered after being molded, it becomes a sintered body having a moderate porosity. When the sintered body is subjected to chemical formation at a predetermined voltage, a dielectric layer having a uniform thickness depending on the applied voltage is formed.
A semiconductor layer laminated on the dielectric layer is formed by chemical and/or electrochemical polymerization by methods such as immersing a sintered body having a dielectric layer formed thereon, which serves as an anode body, in a chemical solution which forms a semiconductor layer.
In anticipation of chemical or physical degradation of the dielectric layer at the time of forming a semiconductor layer in the above described process, a method of forming a thick dielectric layer in advance has been known.
JP 2008-166851 A (U.S. Pat. No. 7,349,198; Patent Document 1) discloses a solid electrolytic capacitor obtained by sequentially forming a first dielectric layer mainly comprising niobium and oxygen, a second dielectric layer containing phosphorus or sulfur, and a cathode on an anode body.
JP 2010-232699 A (U.S. Pat. No. 7,206,192; Patent Document 2) discloses a solid electrolytic capacitor obtained by sequentially forming a first dielectric layer formed by oxidizing a part of an anode body, a second dielectric layer formed on the first dielectric layer, and a cathode on an anode body; in which the oxygen concentration in the second dielectric layer gradually decreases from the first dielectric side to the cathode side.
However, in either method, a dielectric layer becomes thicker as a whole and therefore the decrease in the capacitance with the increased thickness cannot be avoided.